The limited supply and increasing cost of crude oil has prompted the search for alternative processes for producing hydrocarbon products. One such process is the conversion of oxygen-containing (for example methanol), halogenide-containing or sulphur-containing organic compounds to hydrocarbons, in particular, to light olefins, i.e. C2 to C4 olefins, or gasoline and aromatics. In the present application the conversion of said oxygen-containing (also referred to as oxygenates), halogenide-containing or sulphur-containing organic compounds to hydrocarbons, especially light olefins, is referred to as the XTO process. The interest in the XTO process is based on the fact that feedstocks, especially methanol can be obtained from coal, biomass, hydrocarbon residues, petcoke, organic waste or natural gas by the production of synthesis gas, which is then further processed to produce methanol. The XTO process can be combined with an OCP (olefin cracking process) process to increase production of olefins. The XTO process produces light olefins such as ethylene and propylene, as well as heavy hydrocarbons such as butenes and above. These heavy hydrocarbons are cracked in an OCP process to give mainly ethylene and propylene.
In accordance with U.S. Pat. No. 5,573,990 methanol and/or dimethylether is converted to light olefins in the presence of a catalyst, which contains at least 0.7% by weight of phosphorus and at least 0.97% by weight of rare earth elements, which are incorporated within the structure of the catalyst and allegedly enhance the hydrothermal stability of the zeolite. The rare earth elements are preferably rich in lanthanum, the content of lanthanum in the catalyst being preferably comprised between 2.5 and 3.5% by weight of the catalyst. The rare earth elements are introduced via impregnation of the crystal structure with an aqueous solution of a lanthanum salt, for example La(NO3)3, or of mixed rare earth salts rich in lanthanum. The zeolite ZSM-5 based catalyst presents a mole ratio SiO2/Al2O3 comprised between 40 and 80, a crystal size comprised between 1 and 10 μm and adsorption capacities of n-hexane and water of from 10 to 11% by weight and of from 6 to 7% by weight respectively. Said ZSM-5 is synthesized in the presence of a template, then extruded with colloidal silica and converted to the hydrogen form by ion exchange using hydrochloric acid.
US 20060144759 A1 is related to the production of ethylene and propylene from the catalytic cracking of hydrocarbons, which may include an unsaturated bond, but no mention is made of oxygen-containing feedstocks. The aim was to find a catalyst, which could be used in a reactor permitting continuous regeneration of the catalyst. The zeolite thus cited as suitable is a high silica zeolite, preferably a ZSM-5 and/or a ZSM-11, having a SiO2/Al2O3 molar ratio ranging from 25 to 800 and carrying a rare earth element preferably chosen from lanthanum, cerium, praseodymium, neodymium, samarium, gadolinium and dysprosium. It is stated that mere physical mixing of the zeolite with the rare earth compound is not sufficient. The zeolite may also contain other components such as an alkali metal, an alkaline earth metal, a transition metal, a noble metal, a halogen and phosphorus.
In accordance with US 2007/0032379 A1, an alkaline earth metal-containing MFI zeolite is disclosed, having a Si/Al atomic ratio of from 30 to 400, an alkaline earth metal/Al atomic ratio ranging from 0.75 to 15, and an average particle diameter ranging from 0.05 to 2 μm. This zeolite is selective for the production of lower hydrocarbons, e.g. ethylene and propylene, from dimethyl ether and/or methanol and is stated to have an extended catalyst life. The zeolite is obtained by synthesising a zeolite raw material solution, which contains a SiO2 source, a metal oxide source, an alkali source and a structure directing agent, i.e. a template, in the presence of an alkaline earth metal salt, such as calcium acetate, and a zeolite seed crystal. This implies that the metal salt is present within the zeolite crystal structure.
According to U.S. Pat. No. 4,049,573, a catalytic process is provided for converting lower monohydric alcohols to a hydrocarbon mixture rich in ethylene and propylene and mononuclear aromatics with a high selectivity for para-xylene, using a catalyst comprising a crystalline aluminosilicate zeolite having a silica to alumina ratio of at least about 12, a constraint index in the range of 1 to 12, said catalyst having been modified by the addition thereto of a minor proportion of an oxide of boron or magnesium either alone or in combination, optionally with an oxide of phosphorus. The zeolite can be ion-exchanged to form metal-modified zeolites for example with nickel, zinc, calcium or rare earth metals.
In accordance with U.S. Pat. No. 3,911,041, methanol or dimethyl ether is subjected to conversion, at a temperature of at least about 300° C., with a catalyst comprising a crystalline aluminosilicate zeolite having a silica to alumina ratio of at least about 12, a constraint index of about 1 to 12, and containing phosphorus incorporated within the crystal structure thereof in an amount of at least about 0.78 percent by weight, preferably not higher than about 4.5 percent by weight. The zeolite, preferably, also has a dried crystal density of not less than about 1.6 grams per cubic centimeter. The crystalline aluminosilicate zeolite is first converted to the hydrogen form, then phosphorus is introduced by reaction with a phosphorus-containing compound having a covalent or ionic constituent capable of reacting or exchanging with a hydrogen ion. Thereafter, the phosphorus-modified zeolite is heated. There is no steaming of the zeolite prior to introduction of phosphorus. Preferably, prior to reacting the zeolite with the phosphorus-containing compound, the zeolite is dried, preferably in the presence of air and at an elevated temperature. The phosphorus-containing zeolite thus obtained may be further modified by impregnating the zeolite with zinc. This can be carried out by contacting the zeolite with a solution of a zinc salt, so that the zinc salt can fill the pore volume of the phosphorus-containing zeolite. Zinc-impregnated phosphorus-containing zeolites are claimed to have higher levels of conversion than those zeolites not impregnated with zinc.
Sano et al. (Applied Catalysis, 33, 1987, 209-217) discusses the differences of Ca—H-ZSM-5, CaCO3/Ca—H-ZSM-5 and CaO/Ca—H-ZSM-5. The Ca—H-ZSM-5 zeolite was obtained by mixing aluminium nitrate, colloidal silica and calcium acetate, template and sodium hydroxide in solution. Thus, the calcium is contained within the crystal structure of the zeolite. After crystallisation of the zeolite from the hydrogel, the crystals were filtered off and then washed, dried, calcined at 500° C. for 16 hours, protonated and calcined again at 500° C. for 6 hours to obtain CaCO3/Ca—H-ZSM-5. To obtain CaO/Ca—H-ZSM-5, the CaCO3-containing catalyst was calcined once more for a further 24 hours at 600° C. The catalyst stabilities and long-term aging of Ca—H-ZSM-5, CaCO3/Ca—H-ZSM-5 and CaO/Ca—H-ZSM-5 were then compared in methanol conversions. Very slow decays of conversion and selectivity were observed for the CaCO3/Ca—H-ZSM-5 and the CaO/Ca—H-ZSM-5 zeolites. However Ca—H-ZSM-5 decayed rapidly, which is claimed to be due to the increased coke deposition on the catalyst surface. The amount of coke deposited on the CaCO3/Ca—H-ZSM-5 and the CaO/Ca—H-ZSM-5 zeolites was far less. On the other hand, the modification of the calcium-containing catalyst to a CaCO3- or CaO-containing catalyst did not seem to affect resistance to steaming. Thus, the extended catalyst life was attributed to the improved resistance to coking and not to the improved resistance to hydrothermal treatment. All of the H-ZSM-5 disclosed by Sano et al. are phosphorous free.
WO2007/043741 discloses a catalyst for producing light olefins from a hydrocarbon feedstock wherein the catalyst consists of a product obtained by the evaporation of water from a raw material mixture comprising 100 parts by weight of a molecular sieve with a framework of Si—OH—Al groups, 0.01-5.0 parts by weight of a water-insoluble metal salt, and 0.05-17.0 parts by weight of a phosphate compound. Thus the phosphorus and the metal salt compound are added simultaneously. It is stated that the metal salt compound thereby stabilises the phosphate ion species without ion exchange with the protons of the molecular sieve. The water-insoluble metal salt is a metal salt with a solubility product (Ksp) of less than 10−4, i.e. a pKsp of more than 4. This includes oxides, hydroxides, carbonates or oxalates of metals with an oxidation state of more than +2, preferably alkaline earth metals (Mg, Ca, Sr, and Ba), transition metals (Ti, V, Cr, Mn, Fe, Co, Ni, and Cu) and heavy metals (B, Al, Ga, In, Ti, Sn, Pb, Sb and Bi). There is also no indication that this catalyst can be used in XTO processes.
WO2006/028333 discloses a porous solid acid catalyst for producing light olefins from hydrocarbon feedstocks. The catalyst is prepared by carrying out a pillaring reaction of a raw material mixture comprising 42-60 wt % HZSM-5 having a Si/Al molar ratio of 15-300, 12-38 wt % layered compound, 1-20 wt % Al2O3 as a pillaring agent, 1-4 wt % P2O5, 10-15 wt % SiO2 and 0.5-2.5 wt % B2O3 based on an oxide form in water.
Fujisawa et al. (Bull. Chem. Soc. Jpn., 60, 1987, 791-793) discusses the production of light olefins from methanol using phosphorous free H-ZSM-5 zeolites containing alkaline earth metals. The alkaline earth metals were added to the zeolites in the form of their water-soluble acetates.
According to U.S. Pat. No. 4,544,793, a synthetic zeolite for converting methanol and/or dimethyl ether into lower olefins was obtained with a phosphorous free crystalline aluminosilicate having the empirical formula xM2O.yM′O.Al2O3.zSiO2.nH2O wherein M is an exchangeable cation selected from the group consisting of alkali metals, hydrogen and mixtures thereof, M′ is an alkaline earth metal and x is between 0 and 1.5, y is between 0.2 and 40, z is between 12 and 3000 and n is between 0 and 40, wherein x+y is 1.2 or more, and the aluminosilicate has a specific X-ray diffraction pattern.
Metal modified zeolites, particularly, P-zeolites and their use as XTO catalysts are known in the art. Typically, non-dealuminated zeolites obtained by direct synthesis were modified with P by impregnation techniques and then promoted with rare earth elements or Mg. This modification aims to additionally stabilize the phosphorous on the zeolite by means of formation of complex metal-aluminophosphates. These species are more resistant in a hydrothermal environment and protect the aluminium located within the framework against migration.
On the other hand, Ca (calcium) is more typically used as a promoter for phosphorous-free high silica zeolites. This metal is usually not used for P-modified molecular sieves due to very high affinities that Ca exhibits towards P. Typically, calcium interacts mostly with the acid sites in the proximity of the external surface of the zeolite, because of the high diffusion constraint. Higher affinity of Ca towards P in respect of aluminium leads to removal of the phosphorous bound initially to aluminium. This effect leads to recovery of phosphorus free aluminium acid sites, which are far less hydrothermally stable and could promote unwanted side reactions. Therefore, calcium cannot be applied in the case of P-modified molecular sieves in the same manner as, for example, Mg and La, or as calcium was previously used in P-free zeolites. This Ca-comprising compound having Ca present in equal or excess amounts with respect to phosphorous would normally provoke the movement of far too much phosphorous from the microporous structure to the external surface. Therefore, it is important that Ca atoms are saturated with phosphorous.
Thus, use of calcium as a promoter for P-modified molecular sieves requires a special approach. In addition, this approach could be expanded to other alkali-earth metals and also rare-earth metals. This invention proposes a different solution for the preparation of alkaline earth or rare earth metal-P-modified molecular sieves (M-P-modified molecular sieves) consisting in introducing the metal in the form of an alkaline earth or rare earth metal-containing solution (M-containing solution) in the presence of an excess amount of phosphorous in the mixture. The final molar M/P ratio in M-P-zeolite should be lower than 1 and preferably the concentration of the M-containing solution should be at least 0.05-M. The catalyst prepared this way shows a very good performance in XTO and/or OCP processes and provides a superior hydrothermal stability in comparison with M-free P-zeolite. Especially preferred are phosphorus-modified (P-modified) molecular sieves prepared based on zeolites with a low Si/Al ratio, subjected to dealumination by steaming and leaching/P-modification followed by alkaline earth or rare earth metal modifications. Prior dealumination and chemical interaction of aluminum with phosphorus and alkaline earth or rare earth metal inhibit the further dealumination of zeolites, which, in turn, increases their stability and selectivity in XTO.
Thus, the current invention proposes an improved catalyst for XTO and/or OCP processes.
It is thus an aim of the invention is to find a catalyst for XTO and/or OCP processes with an increased yield of light olefins.
It is another aim of the invention to find a catalyst for XTO and/or OCP processes with a higher hydrothermal stability.
In addition, it is another aim of the invention to find a catalyst for XTO and/or OCP processes with reduced selectivity for paraffins.
The invention fulfils at least one of the above aims.